Sound absorption in Hilbert fractal and coiled acoustic metamaterials

G. Comandini,C. Khodr,F. Scarpa,V. P. Ting,M. Azarpeyvand

doi:10.1063/5.0079531

Abstract

We describe here a class of acoustic metamaterials with fractal Hilbert space-filling and coiled geometry with equal tortuosity for noise mitigation. Experiments are performed using a four-microphone impedance tube and benchmarked against non-viscous and viscothermal finite element models related to configurations spanning up to five fractal/geometry orders. We show that the acoustic absorption can be predicted by the resonance of the cavities associated with the tortuous paths. For a given fractal/geometry order, the acoustic absorption at specific frequencies is also enhanced by maximizing the difference between the minimum and maximum fluid particle velocity of the air inside the patterns. These principles can be used to design high-performance acoustic metamaterials for sound absorption over broad frequency ranges.

Highlights

Periodic sub-wavelength structures that modify the phase v/ and group vg velocities of sound.[2,3] It is possible to design metamaterials with null or negative density q and bulk modulus j,4,5 and those anomalous properties are associated with wave phenomena, such as acoustic cloaking[6,7] (q 1⁄4 0), super-lenses[8,9] (q, j < 0), and sound slowness[10,11] (j 1⁄4 0)
Experiments are performed using a four-microphone impedance tube and benchmarked against non-viscous and viscothermal finite element models related to configurations spanning up to five fractal/geometry orders
For a given fractal/geometry order, the acoustic absorption at specific frequencies is enhanced by maximizing the difference between the minimum and maximum fluid particle velocity of the air inside the patterns

Summary

Introduction

Electronic mail: codor.khodr@ec-lyon.fr c)Electronic mail: v.ting@bristol.ac.uk d)Electronic mail: m.azarpeyvand@bristol.ac.uk e)Electronic mail: f.scarpa@bristol.ac.uk For a given fractal/geometry order, the acoustic absorption at specific frequencies is enhanced by maximizing the difference between the minimum and maximum fluid particle velocity of the air inside the patterns.

Results

Conclusion